Eur J Nucl Med Mol Imaging (2014) 41:2291–2293 DOI 10.1007/s00259-014-2924-2

EDITORIAL COMMENTARY

Are SPECT measurements of myocardial blood flow and flow reserve ready for clinical use? Ernest V. Garcia

Published online: 14 October 2014 # Springer-Verlag Berlin Heidelberg 2014

The well-documented diagnostic accuracy of SPECT myocardial perfusion imaging (MPI) for detecting coronary artery disease (CAD) has promoted its widespread clinical use. Nevertheless, the success of SPECT MPI has been attained using the basic SPECT camera design which is over 50 years old [1], using the basic filtered backprojection reconstruction algorithm which is even older, dating to over 90 years ago [2], and using 99mTc-based perfusion agents with limited extraction fractions [3]. Moreover, although SPECT MPI is inherently a digital quantitative technique, our clinical approach to quantifying hypoperfusion has depended on a database approach where a patient’s left ventricular perfusion pattern is statistically compared to an expected normal perfusion pattern generated from patients with a low likelihood of CAD [4–6]. In spite of the success of SPECT MPI using these established conventional approaches, recent innovations are poised to bring SPECT MPI to the next level. In a recent article in EJNMMI, Hsu et al. [7] report on the use of dynamic SPECT with a SPECT/CT camera to measure absolute myocardial blood flow (MBF) and MBF reserve (MBFR) in patients. The authors conclude that their flow quantitation method is a clinically effective approach to enhancing CAD detection. Thus it is pertinent to ask: are SPECT measurements of MBF and MBFR ready for clinical use? Perhaps in a patient-centered environment the first pertinent question should be: how will these measurements help our patients? It has been posited that measurements of MBF and MBFR should not be limited to the role of gatekeeper to the catheterization laboratory but should be used in the more important role of gatekeeper to revascularization [8]. So this approach should not only limit the referral of the 60 % of

E. V. Garcia (*) Department of Radiology and Imaging Sciences, Emory University, 101 Woodruff Circle, Room 1203, Atlanta 30322, GA, USA e-mail: [email protected]

patients who are found not to have obstructive disease in the catheterization laboratory [9] but should also, in the 40 % with obstructive disease, guide the interventionist as to which vessels are truly flow-limiting and have the potential for successful revascularization. As such, the measurements fulfill the mantra of today, i.e., an imaging test should not only yield a correct diagnosis but should also guide successful therapy and thus be directly associated with a patient’s outcome that can be used as evidence of the value of the test. The next pertinent question should be: with today’s SPECT instrumentation, radiopharmaceuticals and quantification software, are the measurements of absolute MBF and MBFR accurate and reproducible enough for clinical use? Limited by the lack of dynamic SPECT, early investigations of MBFR measurements with SPECT used first-pass dynamic planar imaging of tetrofosmin [10] and sestamibi [11] to record the input function followed by SPECT without attenuation correction for myocardial sampling. A simple microsphere model was used to measure MBF and/or MBFR. Dynamic SPECT and compartmental modeling were also used to investigate imaging with teboroxime [12] albeit, at the time, there were very few SPECT cameras that could perform this fast dynamic acquisition. These investigations had in common that the methodology was applied to small patient populations or in animal experiments. All of these studies showed the feasibility of measuring MBF and particularly MBFR with SPECT. In recent years manufacturers have begun to break away from the conventional SPECT imaging approach to create innovative designs for dedicated cardiac imagers. The designs of these imagers have in common that all available detectors are constrained to imaging just the cardiac field of view. These new designs vary in the number and type of scanning or stationary detectors, and whether NaI or solid-state detectors are used [13, 14], but have in common an increase in count sensitivity over conventional SPECT up to a factor of 10 [15]. Similarly, iterative reconstruction has significantly evolved

2292

allowing the physical correction of scatter, attenuation, resolution changes with depth, and image noise. Although these might sound familiar, today’s techniques are more accurate than those used even 10 years ago. Now instead of implementing simple assumptions as in the past, the entire imaging process is modeled to better correct for these physical phenomena. Two groups have reported on the feasibility of using two of these new heart-centered CZT SPECT detector systems to measure MBF [16] and MBFR [16, 17]. Hsu et al. [7], in this issue of EJNMMI, report on the use of dynamic SPECT with a SPECT/CT camera and iterative reconstruction with comprehensive correction methodology to measure absolute MBF and MBFR in humans. These investigators chose to use a standard dual-detector camera equipped with parallel-hole collimators. Somewhat less conventional was the fact that the camera was able to perform a 180° arc acquisition, back and forth, every 10 s to record the input function as the first pass of the sestamibi tracer through the ventricles. Also not typical was their use of iterative reconstruction with comprehensive correction methodology which corrected for scatter, attenuation, resolution changes with depth, and image noise. The approach of these investigators was inspired by the success of PET measurements of MBF and MBFR but motivated by the need to provide flow measurements in situations in which PET and solid-state SPECT are not available. They also chose to use the FlowQuant program developed at the University of Ottawa validated for measuring flow with 82Rb [18] but modified to use a single compartment model and the sestamibi extraction fraction. To determine if a diagnostic method is ready for clinical use, the simple but pertinent question is whether, for a given clinical application in a given patient, the error of the measurement is such that we can reasonably separate, in this case, normal from abnormal MBF and/or MBFR. Hsu et al. [7] report that there was a statistically significant difference in stress MBF and MBFR between their 13 patients with CAD and 8 patients with no significant lesions (p=.02 and p

Are SPECT measurements of myocardial blood flow and flow reserve ready for clinical use?

Are SPECT measurements of myocardial blood flow and flow reserve ready for clinical use? - PDF Download Free
95KB Sizes 0 Downloads 9 Views